Electrical discharge machines are typically divided into a mechanical device featuring a member that supports the work piece and a mechanical member that moves the working electrode and the work piece relative to one another, and a power supply device that is invariably located at a physical distance from the mechanical device and that generates a power pulse. The power supply device generally includes an electrical circuit that generates a power pulse, a control device that controls the amount of energy in the power pulse and its ON and OFF tines, and a control device that controls the relative movement of the working electrode and work piece with respect to one another. The power supply device is connected by a suitable conductor to the mechanical device bearing the work piece and working electrode. Some of the elements which comprise the electrical discharge circuit are physically positioned on the mechanical device, e.g., the lead wires and the conductive members connecting the lead wires and electrode. These elements may be regarded as part of the power supply device.
Such an electrical discharge machine may feature a known power supply system of a type that supplies an AC voltage to the machining gap to machine the work piece.
Particularly in machining process where a water-base machining fluid is used, electrical discharge machines that employ an AC power pulse to machine the work piece enable the average voltage at the machining gap to be brought to substantially zero, making it possible to prevent electrolysis that corrodes the machined surface.
Power supply systems that supply AC voltage include systems that operate by converting a high-frequency DC pulse into an AC pulse, and supplying the high-frequency AC pulse to the machining gap. In concrete terms, these systems are provided with at least a DC power supply and a switching element, and means for turning the switching element ON and OFF in response to a specified signal from a pulse emitting device so as to generate a continuous high-frequency DC pulse having a specified ON time and OFF time. The high-frequency DC voltage is converted into an AC pulse by means of a transformer, and the high-frequency AC pulse is supplied to the machining gap.
Although this type of device will deliver comparatively small amounts of energy in the individual power pulses when used to supply several .mu.s (microsecond)-long power pulses of AC current per cycle, it can still deliver sufficiently large amounts of energy in a given unit time. This principle allows machining to be carried out quickly without compromising the desired surface roughness. Moreover, wire-cut electrical discharge machines that use a wire as the working electrode to cut the work piece can furnish a large voltage at the wire electrode regardless of the small amounts of energy per cycle, enabling a reduction in the amplitude of wire electrode oscillation. As a result, such devices can be used to machine work pieces to a better configurational accuracy and a lower roughness on the machined surface.
Despite such advantages, however, electrical discharge machines that supply an AC pulse to machine the work piece have the drawback of frequently allowing the electrode material (depending on the electrode and work piece material) to stick to the work piece and coat its surface. This coating is particularly marked when the work piece material is tungsten carbide containing cobalt (WC--Co) or copper (Cu), and the electrode material is copper (Cu) or brass (Cu--Zn). In order to obtain a finished product from the work piece, the surface must be ground or subjected to other treatment to remove this coating. A thick coating of electrode material on the surface of the work piece can sometimes prevent the desired surface roughness from being obtained.
In the past, the AC voltage pulse waveforms provided at the machining gap by power supply systems that convert a DC pulse into an AC pulse were regarded as being substantially sinusoidal. However, under careful study, the waveform of such AC voltage pulses was revealed to be formed such that the potential of the work piece with respect to the electrode was biased to a negative polarity. For ease of explanation, when the work piece is positive and the electrode negative, the voltage will be designated as being positive, and as being negative in the opposite situation.
Even if a sinusoidal voltage pulse is supplied, this undesirable coating frequently occurs. Accordingly, even when a sinusoidal AC voltage pulse waveform is obtained in this type of electrical discharge machine by adjusting the ON and OFF timing of the switching element by a frequency corresponding to a constant of the power supply circuit so as to shape the voltage waveform of the AC pulse, the problem of undesirable coating remains difficult to solve. Moreover, simply performing such an operation with this type of device is difficult in and of itself.
In order to solve this problem, the present inventors have supplied an AC voltage pulse having a waveform biased to a positive polarity to carry out repeated machining of-numerous work pieces, and found that this undesirable coating was virtually eliminated.
On the other hand, when the work piece material was e.g. steel (Fe), the absence of coating was marked. When a high-frequency AC voltage pulse biased to a positive polarity was then supplied and a steel work piece was machined, there was an even greater effect on the roughness of the machined surface.
The present invention has been conceived in the light of the above situation, and has the purpose of providing a power supply system that is constituted so as to supply an AC pulse to the machining gap, that is particularly suited devices that convert a DC pulse into an AC pulse and supply the AC pulse to the machining gap, and that is capable of preventing the undesirable coating of electrode material that can stick to the surface of the work piece.
Moreover, the purpose of the present invention is to provide a power supply system for electrical discharge machines with which the waveform of the high-frequency AC pulse supplied to the machining gap can be selectively biased in response to machining conditions that include differences in work piece materials to a positive polarity or a negative polarity, viewed in terms of the potential of the work piece with respect to the electrode, to machine the work piece.
Other purposes of the present invention will be discussed in part in the following explanation, and will in part be revealed by means of the described embodiments to those skilled in the art.